Ultra-compact room temperature rapidly tunable infrared sources

ABSTRACT

An ultra-compact, room-temperature, continuous-wave infrared source that can rapidly tune over a very wide spectral range. The targeted spectral overages are 3 μm to 4 μm, 4 μm to 6 μm, 6 μm to 8 μm, and 8 μm to 12 μm. The spectral width of the infrared idler is expected to be ˜10 MHz. In particular, the invention is a monolithically integrated device which requires no external pump lasers or bulk optics for its operation. The invention uses difference-frequency-generation in a highly nonlinear optical semiconductor waveguide with which high power semiconductor lasers are integrated to internally provide the tunable pump and signal waves.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application Ser.No. 60/185,643 filed on Feb. 29, 2000 and incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTREFERENCE TO A MICROFICHE APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to infrared sources, and moreparticularly to an ultra-compact, room temperature, continuous-wave,rapidly tunable infrared source.

2. Description of the Background Art

Tunable sources in the infrared spectral region are important for anumber of applications, including remote sensing of trace gases. Compactinfrared sources operating at room temperature are especially attractivefor demanding Department of Defense (DoD) operations requiring highmobility and reliability on both ground-based and aircraft platforms. Inmany cases, the infrared sources must be able to rapidly tune over awide spectral range to achieve high-speed measurements on many tracegases.

Although recent research on Quantum Cascade lasers, Type-II lasers, andSolid-state Optical Parametric Oscillators (OPO) have demonstratedimpressive results, none of those infrared sources currently offer thedesired characteristics listed above. Both Quantum Cascade lasers andType-II lasers show high power continuous wave operations at cryogenictemperatures; however, their performance degrades at higher temperaturesand, further, they cease to operate continuous-wave (CW) at roomtemperature. Their tunability is also limited by narrow spectral widths,and only relatively slow thermal tuning have been demonstrated.Solid-state OPOs, on the other hand, perform very well at roomtemperature; however, they require sophisticated bulk-optics alignmentbetween the diode pump laser, the solid-state laser, and the OPOresonator. In addition, their line widths tend to be too broad toresolve dense trace gas lines, and they cease to operate unless the pumppower levels are maintained above the threshold in the multiple wattlevels.

Accordingly, there is a need for a continuous-wave infrared source thatis ultra-compact, rapidly tunable, and operable at room temperature. Thepresent invention satisfies those needs, as well as others, andovercomes deficiencies inherent in conventional infrared sources.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises an ultra-compact, room-temperature,continuous-wave infrared source that can rapidly tune over a very widespectral range. More particularly, the invention comprises amonolithically integrated device which requires no external pump lasersor bulk optics for its operation. Accordingly, an aspect of theinvention is difference-frequency-generation in a highly nonlinearoptical semiconductor waveguide with which high power semiconductorlasers can be integrated to internally provide the tunable pump andsignal waves.

By way of example, and not of limitation, the invention can achieve awide tuning range covering one or multiples of the following approximateinfrared ranges: 3 μm to 4 μm, 4 μm to 6 μm, 6 μm to 8 μm, and 8 μm to12 μm. The spectral width of the infrared idler is approximately 10 MHz.The invention employs zinc-blende semiconductors such as GaAs and InPwhich are excellent nonlinear optical materials as well as superb lasermaterials. They offer high nonlinear optical susceptibilities (e.g. χ⁽²⁾_(GaAs)=180 pm/V), wide transparency in the infrared (approximately 0.9μm to approximately 14 μm for GaAs), high damage threshold(approximately 10 MW/cm², inside the facet), and single mode lasing atpower levels above approximately 1.0 W. For efficient nonlinear opticalprocesses, advanced material processes, including wafer-scale bondingand atomic-scale planarization, are employed to realize a semiconductorwaveguide with patterned crystal orientation for quasi-phasematchedinteractions. Further, the invention exhibits idler tuning over a widespectral range by tuning the two laser emission wavelengths.

An object of the invention is to realize an infrared source operating atroom temperature and capable of rapidly tuning over a wide wavelengthrange in an extremely compact and robust device structure.

Another object of the invention is to provide a device that will operateat room temperature, tune over wide infrared wavelengths, and be housedin a self-contained compact package.

Another object of the invention is to provide a compact integratednonlinear device that achieves room temperature CW operation.

Another object of the invention is to provide a compact integratednonlinear go device that achieves tunable infrared emission covering thefollowing approximate wavelength ranges: 3 μm to 4 μm, 4 μm to 6 μm, 6μm to 8 μm, and 8 μm to 11 μm.

Another object of the invention is to provide a compact integratednonlinear device that achieves a rapid tuning speed of severalnanoseconds for tuning from one desired wavelength to another.

Another object of the invention is to provide a compact integratednonlinear device in a compact device module of approximately 1 cm inlength, which contains tunable pump and signal lasers, a MMI coupler,and a nonlinear waveguide.

Another object of the invention is to provide a compact integratednonlinear device for approximately 3 μm to approximately 4 μm onwavelengths that achieves an approximate 100 mW infrared output ifapproximately 1 W pump and approximately 1 W signal waves are coupledinto the waveguide with a loss coefficient of approximately 2 dB/cm atevery wavelength.

Another object of the invention is to provide a compact integratednonlinear device for approximately 8 μm to approximately 11 μmwavelengths that achieves an approximate 26 mW infrared output ifapproximate 1 W pump and approximate 1 W signal waves are coupled intothe waveguide with a loss coefficient of approximately 2 dB/cm at everywavelength.

Another object of the invention is to provide a room-temperature,continuous-wave tunable infrared source that can rapidly (e.g., lessthan approximately 10 nanoseconds) tune over a very wide spectral range(greater than approximately 25% of the center wavelength).

Another object of the invention is to provide a continuous-wave tunableinfrared source having approximate spectral coverage ranges from 3 μm to4 μm, 4 μm to 6 μm, 6 μm μm to 8 μm, and 8 μm to 12 μm.

Another object of the invention is to provide a continuous-wave tunableinfrared source wherein the spectral width of the infrared idler isapproximately 10 MHz.

Another object of the invention is to provide a monolithicallyintegrated infrared source which requires no external pump lasers orbulk optics for its operation.

Another object of the invention is to provide fordifference-frequency-generation in a highly nonlinear opticalsemiconductor waveguide with which high power semiconductor lasers areintegrated to internally provide the tunable pump and signal waves.

Another object of the invention is to provide a highly nonlinear opticalsemiconductor waveguide with which high power semiconductor lasers areintegrated to internally provide the tunable pump and signal waves basedon difference-frequency-generation.

Further objects and advantages of the invention will be brought out inthe following portions of the specification, wherein the detaileddescription is for the purpose of fully disclosing preferred embodimentsof the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic diagram of a quasi-phasematched waveguide withpatterned crystal orientation according to the present invention.

FIG. 2 is a schematic diagram of a quasi-phasematched waveguide withpatterned crystal orientation as shown in FIG. 1, with integrated lasersaccording to the invention.

FIG. 3 is a graph showing parametric tuning curves for a AlGaAswaveguide according to the invention pumped by five different pumpwavelengths (1310 nm, 980 nm, 850 nm, 780 nm, and 670 nm).

FIG. 4 is a graph showing parametric tuning curves for a AlGaAswaveguide according to the invention pumped at 770 nm.

FIG. 5 is a graph similar to FIG. 3, repeated for idler tuning from 8 μmto 11 μm for tuning the pump from 850 nm to 860 nm.

FIG. 6 is a graph similar to FIG. 5 that shows an expanded view for asignal wavelength tuning from 950 nm to 935 nm, tuning parameter tuningfrom 7.5 μm to 9.8 μm.

FIG. 7 is a schematic diagram in cross-section showing two wafers justbefore the wafer bonding step in a fabrication procedure for aquasi-phasematched waveguide with patterned crystal orientation asdepicted in FIG. 1.

FIG. 8 is a schematic diagram in cross-section showing a patternedtemplate formed after bonding of the wafers shown in FIG. 7 and after agrating is lithographically patterned and GaAs and InGaP layers areselectively etched to reveal a GaAs surface from the bottom substrate.

FIG. 9 is a schematic diagram in cross-section showing aquasi-phasematched waveguide with patterned crystal orientation asdepicted in FIG. 1 after growth of a superlattice layer forplanarization and a waveguide comprising a lower Al_(0.6)Ga_(0.4)Ascladding layer, an Al_(0.5)Ga_(0.5)As core layer, and anAl_(0.6)Ga_(0.4)As on the template shown in FIG. 8.

FIG. 10 is a graph showing an experimentally measured spectrum forsimultaneous conversion of two input channels at 1528 nm and 1536 nm andconverted waves at 1548 nm and 1556 nm for a quasi-phasematchedwaveguide with patterned crystal orientation according to the presentinvention.

FIG. 11 is a graph showing measured conversion efficiency tuning curvefor two conversion processes, TE conversion to TM and TM conversion toTE for a quasi-phasematched waveguide with patterned crystal orientationaccording to the present invention.

FIG. 12 is a schematic diagram of an embodiment of a high power tunablelaser for integration into the structure shown in FIG. 2.

FIG. 13 is a flow diagram showing an embodiment of a device integrationmethod according to the present invention.

FIG. 14 is a schematic diagram in cross-section of a short periodInAs/GaSb superlattice structure.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to FIG. 1 through FIG. 14 of the drawings,for illustrative purposes the present invention is embodied in theapparatus and methods generally described herein. It will be appreciatedthat the apparatus may vary as to configuration and as to details of theparts, and that the method may vary as to the specific steps andsequence, without departing from the basic concepts as disclosed herein.

1. Difference Frequency Generation in Periodically Domain InvertedWaveguides

Referring first to FIG. 1, an aspect of the invention isdifference-frequency-generation in a highly nonlinear opticalsemiconductor waveguide with which high power semiconductor lasers canbe integrated to internally provide the tunable pump and signal waves.The tunable infrared wave is generated by difference frequency mixing ina semiconductor nonlinear optical waveguide 10 which, in FIG. 1, isshown fabricated on a substrate base 12. The generated infrared wave(idler) has a wavelength corresponding to the frequency differencebetween the pump and the signal frequencies. In other words,1/λ_(i)=1/λ_(p)−1/λ_(s), where λ_(i), λ_(p), and λ_(s), are thewavelengths of the idler 14, pump 16, and signal 18 waves, respectively.

Due to dispersion in the waveguide 10, the phase velocities of theinteracting waves are not matched for efficient nonlinear interactions.The power flow from one wave to the other is determined by the relativephase between the waves, and the differing phase velocities between theinteracting waves lead to an alternation of the power flow. Thealternation of the sign of power flow results in repetitive growth anddecay of the generated idler power along the length of interaction. Thedistance over which the relative phase changes by π is called thecoherence length.

The present invention addresses this inherent mismatching by employing aquasi-phasematching technique. This technique involves repeatedmodulation of the sign of the nonlinear susceptibility after eachpropagation through the coherence length. In crystal optics, reversal ofthe sign of the nonlinear susceptibility can be achieved by inversion ofthe crystallographic orientations.

Therefore, in the present invention, waveguide 10 is fabricated withperiodical patterning of semiconductor crystal orientation forquasi-phasematched nonlinear optical interaction. FIG. 1 illustrates thesemiconductor nonlinear optical waveguide 10 with segments 20 a, 20 b ofalternating patterned crystal orientation. Fabrication of such awaveguide is achieved by applying combined processes of wafer-bondingand epitaxial regrowth as discussed below.

Using such an approach, the present invention achieves non-degeneratedifference-frequency generation of the infrared radiation in thetargeted wavelength ranges of 3 μm to 4 μm, 4 μm to 6 μm, 6 μm to 8 μm,and 8 μm to 12 μm using GaAs substrates. While the wide transparencyrange of GaAs implies that the device can potentially cover the entiremid-IR spectrum of 3 μm to 14 μm, it is difficult to realize a deviceachieving high efficiency over such a wide spectral range due to phasematching and mode-overlap considerations. Therefore, the preferredembodiments of the present invention are devices with optimumperformances in one or a few of the narrower wavelength ranges listedabove.

2. Integrated Lasers and Waveguide

While the device depicted in FIG. 1 achieves the desired IR generationthrough the nonlinear optical interaction between the pump 16 and thesignal 18 waves coupled into the waveguide, improved performance can beachieved from an alternative embodiment wherein the pump and the signallasers are on the same chip as the nonlinear waveguide. FIG. 2 showssuch an integrated device 22. This embodiment of the invention includesa Multi-Mode-Interference (MMI) on-chip coupler 24 for efficientlycoupling emissions from the tunable pump 26 and signal 28 lasers intothe nonlinear waveguide 10.

Benefits of such integration are multi-fold. First, the laser emissionneeds no bulk coupling optics which accompany typical insertion lossesof approximately 4 dB to approximately 6 dB going from a diode laser toa semiconductor waveguide. Since the output power of the idler wave isproportional to the product of the pump and the signal optical powers,the total loss is approximately 8 dB to 12 dB for the generated IR. Onthe other hand, MMI couplers typically achieve approximately 1 dB loss,and the total effective loss becomes approximately 2 dB for the IR idlerwave. Hence, an approximately 6 dB to 10 dB improvement in conversionefficiency in the integrated device shown in FIG. 2 is achieved ascompared to using discrete devices. Second, the integrated device shownin FIG. 2 is mechanically robust and physically compact. The inventiveintegrated device requires no bulk optics alignment nor maintenance ofsuch alignment. This makes a strong contrast to typical bulk OPOsrequiring a careful alignment of a nonlinear crystal, cavity mirrors, aNd:YAG rod, diode lasers, and lenses. Third, room temperature or highertemperature operation can be achieved due to the fact that GaAs or InPbased semiconductor lasers can be utilized to provide the pump 26 andthe signal 28 waves for the semiconductor nonlinear optical waveguide10. For example, GaAs based semiconductor lasers already exist that canoperate at temperatures as high as above approximately 238° C. with T₀above approximately 300° C.

For the desired idler wavelengths in the infrared region, there can bemany combinations of pump and signal wavelengths. In the presentinvention, the pump and signal wavelengths are chosen so that excellentroom-temperature operation can be achieved in semiconductor lasersemitting those wavelengths. Of those wavelengths, the preferredwavelengths are on the order of 780 nm, 850 nm, 980 nm, and 670 nmutilizing GaAs substrates, and on the order of 1310 nm and 1550 nmutilizing InP substrates. Note that, by tuning both the pump and signalwavelengths, a very wide idler tuning range in the infrared can beachieved. For example, 770 nm pump and 990 nm signal waves can mix togenerate a 3.5 μm idler wave, and the tuning of the idler over a rangefrom 3.16 μm to 3.82 μm can be achieved by tuning both the 770 nm and990 nm lasers by +/−10 nm each in the opposite directions. Similarly,780 nm pump and 850 nm signal waves can generate a 10 μm idler wave,which can tune between 7.4 μm and 13.3 μm by tuning both 780 nm and 850nm lasers by +/−10 nm each.

The wavelength tunings described in the examples above induce smallphase-mismatch in the nonlinear interactions due to their deviation fromthe designed quasi-phasematching condition, and this phase-mismatch canbe corrected by modifying the quasi-phasematching condition during thewavelength tuning. Such a modification is possible, for instance, byinjecting a current into the nonlinear waveguide or by applying anelectrical bias. FIG. 3 shows parametric tuning curves of a designedAlGaAs waveguide for five different pump wavelengths (1310 nm, 980 nm,850 nm, 770 nm, and 670 nm), and FIG. 4 shows the tuning curve for a 770nm pump. The abscissa “Tuning” in both figures indicates relative tuningin the injected current or the electrical bias required to maintain thedesired (e.g., “perfect”) quasi-phasematching condition. Note that it isalways possible to utilize an alternate grating periodicity to offsetthe detuning, and the grating period should be chosen to provide thelargest tunability in the desired spectral region.

The rectangular windows 40 and 42 in FIG. 4 also indicate how muchtuning of the signal wavelength is required to achieve the idler tuningbetween 3 μm and 4 μm, provided that the pump wavelength is maintainedat 770 nm. During the course of this idler wavelength tuning from 3 μmto 4 μm, the quasi-phasematching tuning parameter must also tune by 2.8μm to 3.4 μm, which corresponds to tuning of the phase-mismatchparameter Δ  k = (k_(pump)^(TE) − k_(signal)^(TE) − k_(idler)^(TM))

by 0.20 μm⁻¹. This is easily accommodated by either current injection orby reverse biasing the nonlinear optical waveguide. The latter is thepreferred method since it induces negligible excess heating or opticalloss.

In the case of current tuning, this tuning requirement corresponds to areasonable current density of 51 kA/cm² for bulk GaAs, assuming carrierlifetime of 1 nsec, free carrier plasma relations, and typical materialcharacteristics. In this regard, note that the current requirement issignificantly reduced for quantum well structures, so quantum wellstructures that offer a large tuning for the least amount of currentcould be do used as an alternative. In particular, note that this tuningrelates to the phase mismatch term being proportional toΔ  k = (k_(pump)^(TE) − k_(signal)^(TE) − k_(idler)^(TM))

and that both polarization and dispersion must be carefully consideredsince the signal and the pump waves will be in the TE polarization andthe idler in the TM. Therefore, both dispersion and birefringence shouldbe considered in designing the tuning elements. In particular, it may bepossible to employ strained quantum wells for achieving enhancedsensitivities in dispersion or birefringence just below its bandgap.

The tuning by current injection approach offers effective and rapidtuning of the quasi-phasematching condition to cover the wide IRspectrum. However, it may also be possible to employ an alternative fasttuning mechanism such as electro-optical tuning by an external biasfield. Such a method could have the advantage of inducing no excessfree-carrier loss or power dissipation. The electro-optic coefficientsof bulk semiconductors are rather too small to be practical, requiring a600 V bias for the same detuning ofΔ  k = (k_(pump)^(TE) − k_(signal)^(TE) − k_(idler)^(TM))

by 0.20 μm⁻¹. On the other hand, quantum wells offer far more enhancedelectro-optic effect compared to the bulk materials, especially justbelow the bandgap. The estimated required voltage is approximately 30 Vfor the IR tuning of 3 μm to 4 μm. Quantum well designs will achieveidler tuning over a wide spectral range using the minimum bias voltage.

While the foregoing discussion has focused on infrared generation in theapproximate range of 3 μm to 4 μm, devices covering other wavelengthranges such as 4 μm to 6 μm, 6 μm to 8 μm, and 8 μm to 11 μm can bedesigned in a similar manner. FIG. 5 and FIG. 6 show tuning curves for 8μm to 11 μm. FIG. 5 shows the idler tuning from 8 μm to 11 μm for tuningthe pump from 850 nm to 860 nm, and the signal tuning from 950 nm to 935nm is shown in FIG. 6. For this tuning, the relative tuning parametermust also change from 7.5 μm to 9.8 μm to correspond to tuning of thephase mismatch by 0.12 μm⁻¹, and the required current or bias voltagetuning is approximately half of the previous values calculated for 3 μmto 4 μm idler tuning.

3. Fabrication of Periodically Domain Inverted Waveguides

Referring now to FIG. 7, wafer-bonding, selective etching, and epitaxialregrowth are employed to realize a periodically domain invertedwaveguide according to the invention. The design and fabricationtechniques of the periodic-domain-reversed waveguide comprises those ofa template and those of a waveguide. First, the template is prepared bybonding two [001] orientation Metal-Organic Chemical Vapor Deposition(MOCVD) grown wafers such that the [110] crystal directions of the twowafers are parallel to each other. The top wafer 50 comprises a GaAssubstrate layer 52, a 1 μm thick Al_(0.8)Ga_(0.2)As layer 54, a 0.1 μmthick GaAs layer 56, and a 200 Angstrom thick In_(0.5)Ga_(0.5)P layer58. The bottom wafer 60 comprises a GaAs substrate layer 62 and 200Angstrom thick In_(0.5)Ga_(0.5)P layer 64. The wafer bonding processwill achieve an intimate atomic rearrangement at the bonding interface66.

Referring now to FIG. 8, after the wafers are bonded, the upper GaAssubstrate layer 52 and 1 μm Al_(0.8)Ga_(0.2)As sacrificial layer 54 areselectively etched away. Subsequent patterning of a grating andselective wet-etching steps will reveal a patterned template 72 havingalternating GaAs surfaces 68 from the bottom substrate 62 and GaAssurfaces 70 from GaAs epitaxial layer 56 from the removed top wafer 50.Note that the grating lines extend in the [110] direction and thecrystal domains alternate.

Referring to FIG. 9, the nonlinear AlGaAs waveguide 10 of the inventionis then MOCVD grown on the patterned template 72. The waveguidecomprises a planarization layer 74, a lower 2 μm thickAl_(0.6)Ga_(0.4)As cladding layer 76, a 1 μm thick Al_(0.5)Ga_(0.5)Ascore layer 78, and an upper 2 μm thick Al_(0.6)Ga_(0.4)As cladding layer80. Subsequent patterning of a waveguide stripe and Al_(0.6)Ga_(0.4)Asregrowth yields a buried hetero waveguide. Note that the layerthicknesses shown in FIG. 9 are for a 1.5 μm device used as atelecommunications wavelength converter in experiments. For thepreferred infrared wavelengths described above, the layers would bethicker to give longer wavelength idlers.

FIG. 10 and FIG. 11 show experimental results from a device optimizedfor nearly-degenerate difference frequency generation in the 1550 nmregion. In FIG. 10, a pump wave at 771 nm and two input signals at 1528nm and 1536 nm are utilized to generate two idler waves at 1556 nm and1548 nm. FIG. 11 shows the measured conversion efficiency tuning curveas the input signal is tuned from 1490 to 1610 nm. For the fixed pumpwavelength and the QPM grating conditions, the measured bandwidth of thetuning curve exceeds 90 nm. The measured conversion efficiency was arelatively low −17 dB due to large scattering loss in the fabricatedwaveguide. This scattering loss was induced by corrugations in thewaveguide due to imperfect planarization by the superlatticeplanarization layer. A −4 dB conversion efficiency with a reasonableloss level of 3 dB/cm and 100 mW pump power should be achievable.Utilizing an atomic-scale planarization method should significantlyreduce the waveguide corrugation and the scattering loss.

Note that the quality of wafer-bonding is critical to achieving a lowloss waveguide. This is addressed through wafer-scale wafer-bonding in aultra-high vacuum bonding chamber and will be valuable in achieving alow loss waveguide with low defect density. While the wafer-scalebonding is generally very successful, the bonding interface can showair-pockets resulting from trapped gas and moisture. Therefore, by usingan ultra-high vacuum so that there is no possibility of trapping gasesor moisture at the bonding interface.

4. High Power Tunable Lasers

Referring to FIG. 12, the present invention also contemplatesintegrating high-power widely tunable lasers with the nonlinearwaveguide described above. Currently, tunable lasers using sampledgrating Distributed-Bragg-Reflectors (DBR) appear to be suitable forthis purpose. FIG. 12 shows a schematic of a high power tunable laser100 to be integrated with the nonlinear waveguide 10 as part of theintegrated device 22 shown in FIG. 2. This example shows a laser withfront 102 and back 104 sampled grating DBRs, and a tapered amplifiersection 106 for high power. The sampled gratings 102, 104 are fabricatedby deep UV holographic exposure employing phase gratings. Thisfabrication technique will achieving a narrow linewidth lasing action(approximately 10 MHz) of the pump and the signal lasers for narrowlinewidth idler emission in the infrared. In addition, strained quantumwell designs could be employed to further broaden the gain spectralwidth while maintaining superior lasing performance. The output of thetunable laser will include a flared amplifier section 106 to achieveoutput power levels in the 1 W regime and above. The passive taper 108,only a portion of which is shown, is designed to reduce the size of thebeam before it couples into the MMI 24 shown in FIG. 2, and isessentially a reverse process of the beam expansion in the flaredamplifier 106. The self-aligned process described below in connectionwith FIG. 13 is used to align the flared-amplifier 106 and the passivetaper waveguide 108 seamlessly.

5. MMI Coupler

Referring again to FIG. 2, the integrated device 22 shown thereinemploys an MMI 24 to achieve efficient coupling of the pump and thesignal waves from the two tunable lasers 26, 28, respectively, into thenonlinear waveguide 10. In particular, the wavelength dependent MMIcharacteristics are exploited to achieve efficient coupling of both pumpand signal waves into the nonlinear waveguide. For the embodiments ofthe present invention previously described, a 780 nm/980 nm coupler isused for a 3 μm to 4 μm device and a 850 nm/980 nm coupler is used foran 8 μm to 11 μm device. Conventional design procedures known to thoseskilled in the art are employed.

In this regard, note that MMI couplers are reasonably tolerant tofabrication errors and wavelength tunings, and preliminary calculationsshow less than 7% variations in the coupling efficiency for tuning thepump and the signal lasers by ±10 nm. Self-aligned dry etching processesare preferably used to precisely define the MMI coupler, the waveguide,and the tunable lasers. In addition, dry etching allows forming nearlyvertical etched walls for both MMI couplers and waveguides, whichprovides improved mode overlap between interacting waves inside thewaveguide.

6. Integration Process

FIG. 13 schematically shows an embodiment of device integration steps toform the integrated device 22 shown in FIG. 2, which include waferbonding, template fabrication, MOCVD regrowth, self-aligned dry-etching,and metallization. The self aligned dry-etching removes the need toalign the laser sections and the passive waveguide sections. This isimportant especially for the tapered high power laser shown in FIG. 12,since the multi-moded coupling region can cause unintentional multi-modeinterference in the passive waveguide if the laser and the waveguide arenot aligned properly.

In this integration process, dry etching is utilized for realizingvertical walls in the etched laser and waveguide sidewalls necessary forhigh quality integration and MOCVD regrowths. Each dry etching step isfollowed by a wet etching step to remove materials damaged from dryetching processes.

The integration process preferably proceeds follows. First, at step 200the domain inverted template 72 (see FIG. 8) is fabricated on asubstrate employing the procedure for fabricatingperiodically-domain-inverted waveguides shown in FIG. 7 through FIG. 9and described above. Second, at step 202 the pump laser structure 300 isgrown on the template and is partially removed to allow growth of asignal laser structure. Third, at step 204 the signal laser structure302 is grown and is partially removed where the pump laser structure 300exists. Fourth, at step 206 the sequence involves dry etching followedby wet etching of the laser structures where the passive waveguidestructure 304 needs to be grown and then excessive growth on the laserregion is wet etched. Fifth, at step 208 laser 26, 28 and passivewaveguide 306, 308 regions are patterned using a self-aligning maskprocess and are dry etched, and the MMI 24 is deposited. Sixth, at step210 holographic gratings 310, 312 for the sampled grating lasers are bepatterned. In the final step 212, contact layer regrowth andmetallization takes place to deposit contacts 314, 316, 318.

Preferably, the present invention employs a compliant substrate 12fabricated by using the wafer-bonding process. The compliant substratepreferably includes a thin and elastic GaAs membrane on top surface toaccommodate epitaxial growth of zinc-blende semiconductors of anylattice constant. This offers two major improvements for Type-IIdevices. First, GaAs provides far superior surface chemistry compared toGaSb. Second, Type-II quantum wells with various lattice constants canbe investigated for superior performance in both lasers and detectorapplications. Wafer-bonding technology offers a high-quality compliantsubstrate where a thin layer of GaAs membrane can accommodate subsequentepitaxial growth of materials of any lattice constant.

Note that some IR detector work exploits Type-II alignment of InAs/GaSbquantum wells for IR detectors. FIG. 14 shows an example of a Type-IIstructure 400 used for the IR detector application. Short periodsuperlattices of InAs/GaSb lead to the formation of conduction andvalence minibands. In these band states, heavy holes are largelyconfined to the GaSb layers 402 and electrons are primarily confined tothe InAs layers 404. However, because of the relatively low electronmass in InAs, the electron wavefunctions extend considerably beyond theinterfaces and have significant overlap with heavy hole wavefunctions.Hence, significant absorption is possible at the minigap energy (shownin FIG. 14 with the vertical arrow 406) which is tunable by changinglayer thickness. Cutoff wavelengths between approximately 3 μm andapproximately 18 μm are possible with this system. Additionally, sincethe gap of each constituent material is larger than the effective directgap of the superlattice, dark currents are suppressed in comparison withtheir values in similar cutoff wavelength bulk ternary alloys such asHgCdTe. Another benefit of this structure for detector applications isthat normal incidence absorption is permitted by selection rules,obviating the need for grating structures or corrugations which areneeded in quantum well infrared photodetectors (QWIPs). Finally, Augertransition rates, which place intrinsic limits on the performance ofsuch detectors and severely impact the lifetimes found in the bulk,narrow-gap detectors, can be engineered “small” by judicious choices forthe structure's geometry and strain profile.

Although the description above contains many specificities, these shouldnot be construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Thus the scope of this invention should be determinedby the appended claims and their legal equivalents. Therefore, it willbe appreciated that the scope of the present invention fully encompassesother embodiments which may become obvious to those skilled in the art,and that the scope of the present invention is accordingly to be limitedby nothing other than the appended claims, in which reference to anelement in the singular is not intended to mean “one and only one”unless explicitly so stated, but rather “one or more.” All structural,chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present invention, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

What is claimed is:
 1. An infrared source, comprising: a tunable signallaser; a tunable pump laser; and a non-linear optical semiconductorwaveguide coupled to said lasers, said waveguide comprising a pluralityof optical semiconductor waveguide segments having alternating crystalorientation; wherein output wavelength is tunable by tuning at least oneof said signal and pump lasers; wherein said waveguide providesquasi-phasematching of output signals from said signal and pump lasers;and wherein said quasi-phasematching is electrically tunable byinjection of a current into said waveguide or by applying an electricalbias voltage to said waveguide.
 2. An infrared source as recited inclaim 1, wherein pump and signal waves injected into said waveguide mixto generate an idler wave by difference frequency mixing.
 3. An infraredsource as recited in claim 1, wherein said segments comprise GaAs havingalternating reversed [110] crystal directions.
 4. An infrared source asrecited in claim 1, wherein said segments comprise InP havingalternating reversed [110] crystal directions.
 5. An infrared source asrecited in claim 1, wherein at least one segment comprises an upperAl_(0.6)Ga_(0.4)As cladding layer, a Al_(0.5)Ga_(0.5)As core layer, anda lower Al_(0.6)Ga_(0.4)As cladding layer on a GaAs substrate layer. 6.An infrared source as recited in claim 5, wherein said upper claddinglayer is approximately 2 μm thick, said core layer is approximately 1 μmthick, and said lower cladding layer is approximately 2 μm thick.
 7. Aninfrared source as recited in claim 1, wherein at least one segmentcomprises an upper Al_(0.6)Ga_(0.4)As cladding layer, aAl_(0.5)Ga_(0.5)As core layer, a lower Al_(0.6)Ga_(0.4)As claddinglayer, a GaAs layer, an upper In_(0.5)Ga_(0.5)P layer, and a lowerIn_(0.5)Ga_(0.5)P layer on a GaAs substrate.
 8. An infrared source asrecited in claim 7, wherein said upper cladding layer is approximately 2μm thick, said core layer is approximately 1 μm thick, and said lowercladding layer is approximately 2 μm thick.
 9. An infrared source asrecited in claim 7, wherein said GaAs layer is approximately 0.1 μmthick, said upper In_(0.5)Ga_(0.5)P layer is approximately 200 Angstromsthick, and said lower In_(0.5)Ga_(0.5)P layer is approximately 200Angstroms thick.
 10. An infrared source as recited in claim 1, whereinat least one segment comprises an upper Al_(0.6)Ga_(0.4)As claddinglayer, a Al_(0.6)Ga_(0.4)As core layer, and a lower Al_(0.6)Ga_(0.4)Ascladding layer on a GaAs substrate layer, and wherein at least onesegment comprises an upper Al_(0.6)Ga_(0.4)As cladding layer, aAl_(0.6)Ga_(0.4)As core layer, a lower Al_(0.6)Ga_(0.4)As claddinglayer, a GaAs layer, an upper In_(0.5)Ga_(0.5)P layer, and a lowerIn_(0.5)Ga_(0.5)P layer on said GaAs substrate.
 11. An infrared sourceas recited in claim 10, wherein said upper cladding layer isapproximately 2 μm thick, said core layer is approximately 1 μm thick,and said lower cladding layer is approximately 2 μm thick.
 12. Aninfrared source as recited in claim 10, wherein said GaAs layer isapproximately 0.1 μm thick, said upper In_(0.5)Ga_(0.5)P layer isapproximately 200 Angstroms thick, and said lower In_(0.5)Ga_(0.5)Player is approximately 200 Angstroms thick.
 13. An infrared source asrecited in said upper cladding layer is approximately 2 μm thick, saidcore layer is approximately 1 μm thick, said lower cladding layer isapproximately 2 μm thick, said GaAs layer is approximately 0.1 μm thick,said upper In_(0.5)Ga_(0.5)P layer is approximately 200 Angstroms thick,and said lower In_(0.5)Ga_(0.5)P layer is approximately 200 Angstromsthick.
 14. An infrared source, comprising: a tunable signal laser; atunable pump laser; and a non-linear optical semiconductor waveguidecoupled to said lasers, said waveguide comprising a plurality ofperiodic domain inverted semiconductor waveguide segments; whereinoutput wavelength is tunable by tuning at least one of said signal andpump lasers; wherein said waveguide provides quasi-phasematching ofoutput signals from said signal and pump lasers; and wherein saidquasi-phasematching is electrically tunable by injection of a currentinto said waveguide or by applying an electrical bias voltage to saidwaveguide.
 15. An infrared source as recited in claim 14, wherein pumpand signal waves injected into said waveguide mix to generate an idlerwave by difference frequency mixing.
 16. An infrared source as recitedin claim 14, wherein said segments comprise GaAs having alternatingreversed [110] crystal directions.
 17. An infrared source as recited inclaim 14, wherein said segments comprise InP having alternating reversed[110] crystal directions.
 18. An infrared source as recited in claim 14,wherein at least one segment comprises an upper Al_(0.6)Ga_(0.4)Ascladding layer, a Al_(0.5)Ga_(0.5)As core layer, and a lowerAl_(0.6)Ga_(0.4)As cladding layer on a GaAs substrate layer.
 19. Aninfrared source as recited in claim 18, wherein said upper claddinglayer is approximately 2 μm thick, said core layer is approximately 1 μmthick, and said lower cladding layer is approximately 2 μm thick.
 20. Aninfrared source as recited in claim 14, wherein at least one segmentcomprises an upper Al_(0.6)Ga_(0.4)As cladding layer, aAl_(0.5)Ga_(0.5)As core layer, a lower Al_(0.6)Ga_(0.4)As claddinglayer, a GaAs layer, an upper In_(0.5)Ga_(0.5)As layer, and a lowerIn_(0.5)Ga_(0.5)P layer on a GaAs substrate.
 21. An infrared source asrecited in claim 20, wherein said upper cladding layer is approximately2 μm thick, said core layer is approximately 1 μm thick, and said lowercladding layer is approximately 2 μm thick.
 22. An infrared source asrecited in claim 20, wherein said GaAs layer is approximately 0.1 μmthick, said upper In_(0.5)Ga_(0.5)P layer is approximately 200 Angstromsthick, and said lower In_(0.5)Ga_(0.5)P layer is approximately 200Angstroms thick.
 23. An infrared source as recited in claim 14, whereinat least one segment comprises an upper Al_(0.6)Ga_(0.4)As claddinglayer, a Al_(0.5)Ga_(0.5)As core layer, and a lower Al_(0.6)Ga_(0.4)Ascladding layer on a GaAs substrate layer, and wherein at least onesegment comprises an upper Al_(0.6)Ga_(0.4)As cladding layer, aAl_(0.5)Ga_(0.5)As core layer, a lower Al_(0.6)Ga_(0.4)As claddinglayer, a GaAs layer, an upper In_(0.5)Ga_(0.5)P layer, and a lowerIn_(0.5)Ga_(0.5)P layer on said GaAs substrate.
 24. An infrared sourceas recited in claim 23, wherein said upper cladding layer isapproximately 2 μm thick, said core layer is approximately 1 μm thick,and said lower cladding layer is approximately 2 μm thick.
 25. Aninfrared source as recited in claim 23, wherein said GaAs layer isapproximately 0.1 μm thick, said upper In_(0.5)Ga_(0.5)P layer isapproximately 200 Angstroms thick, and said lower In_(0.5)Ga_(0.5)Player is approximately 200 Angstroms thick.
 26. An infrared source asrecited in claim 23, wherein said upper cladding layer is approximately2 μm thick, said core layer is approximately 1 μm thick, said lowercladding layer is approximately 2 μm thick, said GaAs layer isapproximately 0.1 μm thick, said upper In_(0.5)Ga_(0.5)P layer isapproximately 200 Angstroms thick, and said lower In_(0.5)Ga_(0.5)Player is approximately 200 Angstroms thick.
 27. An infrared source,comprising: (a) a tunable signal laser; (b) a tunable pump laser; and(c) means for quasi-phasematching of output signals from said signal andpump lasers, wherein said quasi-phasematching is electrically tunable.28. An infrared source as recited in claim 27, wherein said infraredsource generates an output signal by difference frequency mixing ofinput signals from said pump and signal lasers, and wherein tuning asaid one of said lasers adjusts the wavelength of the output signal. 29.An infrared source as recited in claim 27, wherein at least one of saidlasers comprises a tunable laser with sampled distributed Bragg grating.30. An infrared source as recited in claim 27, wherein said means forquasi-phasematching of output signals from said signal and pump laserscomprises: a non-linear optical semiconductor waveguide; said waveguidecomprising a plurality of optical semiconductor waveguide segmentshaving alternating crystal orientation; wherein said quasi-phasematchingis electrically tunable by injection of a current into said waveguide orby applying an electrical bias voltage to said waveguide.
 31. Aninfrared source as recited in claim 27, wherein said segments compriseGaAs having alternating reversed [110] crystal directions.
 32. Aninfrared source as recited in claim 27, wherein said segments compriseInP having alternating reversed [110] crystal directions.
 33. Infraredsource as recited in claim 27, wherein at least one segment comprises anupper Al_(0.6)Ga_(0.4)As cladding layer, a Al_(0.5)Ga_(0.5)As corelayer, and a lower Al_(0.6)Ga_(0.4)As cladding layer on a GaAs substratelayer.
 34. An infrared source as recited in claim 33, wherein said uppercladding layer is approximately 2 μm thick, said core layer isapproximately 1 μm thick, and said lower cladding layer is approximately2 μm thick.
 35. An infrared source as recited in claim 27, wherein atleast one segment comprises an upper Al_(0.6)Ga_(0.4)As cladding layer,a Al_(0.5)Ga_(0.5)As core layer, a lower Al_(0.6)Ga_(0.4)As claddinglayer, a GaAs layer, an upper In_(0.5)Ga_(0.5)P layer, and a lowerIn_(0.5)Ga_(0.5)P layer on a GaAs substrate.
 36. An infrared source asrecited in claim 35, wherein said upper cladding layer is approximately2 μm thick, said core layer is approximately 1 μm thick, and said lowercladding layer is approximately 2 μm thick.
 37. An infrared source asrecited in claim 35, wherein said GaAs layer is approximately 0.1 μmthick, said upper In_(0.5)Ga_(0.5)P layer is approximately 200 Angstromsthick, and said lower In_(0.5)Ga_(0.5)P layer is approximately 200Angstroms thick.
 38. An infrared source as recited in claim 27, whereinat least one segment comprises an upper Al_(0.6)Ga_(0.4)As claddinglayer, a Al_(0.5)Ga_(0.5)As core layer, and a lower Al_(0.6)Ga_(0.4)Ascladding layer on a GaAs substrate layer, and wherein at least onesegment comprises an upper Al_(0.6)Ga_(0.4)As cladding layer, aAl_(0.5)Ga_(0.5)As core layer, a lower Al_(0.6)Ga_(0.4)As claddinglayer, a GaAs layer, an upper In_(0.5)Ga_(0.5)P layer, and a lowerIn_(0.5)Ga_(0.5)P layer on said GaAs substrate.
 39. An infrared sourceas in claim 38, wherein said upper cladding layer is approximately 2 μmthick, said core layer is approximately 1 μm thick, and said lowercladding layer is approximately 2 μm thick.
 40. An infrared source asrecited in claim 38, wherein said GaAs layer is approximately 0.1 μmthick, said upper In_(0.5)Ga_(0.5)P layer is approximately 200 Angstromsthick, and said lower In_(0.5)Ga_(0.5)P layer is approximately 200Angstroms thick.
 41. An infrared source as recited in claim 38, whereinsaid upper cladding layer is approximately 2 μm thick, said core layeris approximately 1 μm thick, said lower cladding layer is approximately2 μm thick, said GaAs layer is approximately 0.1 μm thick, said upperIn_(0.5)Ga_(0.5)P layer is approximately 200 Angstroms thick, and saidlower In_(0.5)Ga_(0.5)P layer is approximately 200 Angstroms thick. 42.A tunable infrared source, comprising: (a) a tunable signal laser; (b) atunable pump laser; and (c) a non-linear optical semiconductorwaveguide, said waveguide comprising a plurality of opticalsemiconductor waveguide segments having alternating crystal orientation;(d) wherein pump and signal waves injected into said waveguide mix togenerate an idler wave by difference frequency mixing; (e) wherein saidinfrared source generates an output signal by difference frequencymixing of input signals from said pump and signal lasers, and whereintuning a said one of said lasers adjusts the wavelength of the outputsignal; (f) wherein said waveguide provides quasi-phasematching ofoutput signals from said signal and pump lasers; and (g) wherein saidquasi-phasematching is electrically tunable by injection of a currentinto said waveguide or by applying an electrical bias voltage to saidwaveguide.
 43. An infrared source as recited in claim 42, furthercomprising a multi-mode-interference coupler between said waveguide andsaid signal and pump lasers.
 44. An infrared source as recited in claim42, wherein at least one of said lasers includes a sampled distributedBragg grating.
 45. An infrared source as recited in claim 42, whereinsaid segments comprise GaAs having alternating reversed [110] crystaldirections.
 46. An infrared source as recited in claim 42, wherein saidsegments comprise InP having alternating reversed [110] crystaldirections.
 47. An infrared source as recited in claim 42, wherein atleast one segment comprises an upper Al_(0.6)Ga_(0.4)As cladding layer,a Al_(0.5)Ga_(0.5)As core layer, and a lower Al_(0.6)Ga_(0.4)As claddinglayer on a GaAs substrate layer.
 48. An infrared source as recited inclaim 47, said upper cladding layer is approximately 2 μm thick, saidcore layer is approximately 1 μm thick, and said lower cladding layer isapproximately 2 μm thick.
 49. An infrared source as recited in claim 42,wherein at least one segment comprises an upper Al_(0.6)Ga_(0.4)Ascladding layer, a Al_(0.5)Ga_(0.5)As core layer, a lowerAl_(0.6)Ga_(0.4)As cladding layer, a GaAs layer, an upperIn_(0.5)Ga_(0.5)P layer, and a lower In_(0.5)Ga_(0.5)P layer on a GaAssubstrate.
 50. An infrared source as recited in claim 49, wherein saidupper cladding layer is approximately 2 μm thick, said core layer isapproximately 1 μm thick, and said lower cladding layer is approximately2 μm thick.
 51. An infrared source as recited in claim 49, wherein saidGaAs layer is approximately 0.1 μm thick, said upper In_(0.5)Ga_(0.5)Player is approximately 200 Angstroms thick, and said lowerIn_(0.5)Ga_(0.5)P layer is approximately 200 Angstroms thick.
 52. Aninfrared source as recited in claim 42, wherein at least one segmentcomprises an upper Al_(0.6)Ga_(0.4)As cladding layer, aAl_(0.5)Ga_(0.5)As core layer, and a lower Al_(0.6)Ga_(0.4)As claddinglayer on a GaAs substrate layer, and wherein at least one segmentcomprises an upper Al_(0.6)Ga_(0.4)As cladding layer, aAl_(0.5)Ga_(0.5)As core layer, a lower Al_(0.6)Ga_(0.4)As claddinglayer, a GaAs layer, an upper In_(0.5)Ga_(0.5)P layer, and a lowerIn_(0.5)Ga_(0.5)P layer on said GaAs substrate.
 53. An infrared sourceas recited in claim 52, wherein said upper cladding layer isapproximately 2 μm thick, said core layer is approximately 1 μm thick,and said tower cladding layer is approximately 2 μm thick.
 54. Aninfrared source as recited in claim 52, wherein said GaAs layer isapproximately 0.1 μm thick, said upper In_(0.5)Ga_(0.5)P layer isapproximately 200 Angstroms thick, and said lower In_(0.5)Ga_(0.5)Player is approximately 200 Angstroms thick.
 55. An infrared source asrecited in claim 52, wherein said upper cladding layer is approximately2 μm thick, said core layer is approximately 1 μm thick, said lowercladding layer is approximately 2 μm thick, said GaAs layer isapproximately 0.1 μm thick, said upper In_(0.5)Ga_(0.5)P layer isapproximately 200 Angstroms thick, and said lower In_(0.5)Ga_(0.5)Player is approximately 200 Angstroms thick.